gold screws & treated lumber

[Jules]

Hmm, so my box of "gold screws" (which I guess are galvanized) say on the box, "should not be used in treated lumber". Why not? What happens?

cheers

Jules

[Bob F]

They rust away and the project falls apart.

[Tegger]

Jules wrote in news: [e-mail]:

From the Gold Coat FAQ: "In the past few years, the manufactures of wood preservative chemicals changed the formulations they use for most residential and commercial applications. New formulations, including ACQ and copper azole, contain more copper than the historically used CCA. Because dissimilar metals will react with each other, the increase in copper content has created a more corrosive environment for the steel in the structural connectors and fasteners."

[Jules]

Interesting!

After some googling, it sounds like stainless is the best solution, followed by 'hot dipped' fasteners (and I've actually got some hot dipped staples on the shelf which do indeed say they're OK for PT-lumber).

I'll have to see if any of the local supply places have something suitable later on today...

(some sites do seem to suggest that the corrosion's such a slow process that it's not a big deal to use 'regular' fasteners in anything non-structural, and to just replace as/when/if needed, but I think I'd rather do things properly from the outset)

cheers

Jules

[SteveBell]

It's not always a slow process. I've seen drywall screws go in a year or two. Of course, using drywall screws in lumber is dumb, but lots of people do it. I've seen regular screws that didn't last but a few years.

And the regular screws stain the wood.

[willshak]

...and the heads can break off when trying to replace them.

[KC]

"Gold" fasteners are usually cadmium plated and not galvanized. Because they are 'plated' rather than 'dipped", they do not have enough coating to avoid the corrosive effects of ACQ pressure treated lumber.

However MCQ pressure treated lumber is now on the market and is claimed to be much better suited to standard exterior fasteners. As I understand it, the "M" in MCQ stands for 'micronized" meaning that the copper compound is much finer and penetrates the wood much better, therefore they can use less of it. If you look at the wood it only has a slight hint of green copper color vs the predominant green color of ACQ. MCQ lumber is sold by Home Depot and "Yella Wood" distributors. It has a 40yr warranty like ACQ & CCA PT lumber. KC

[DT]

We just replaced the aluminum cladding on several 6x6 porch posts for a customer. They were wrapped in the aluminum coil stock that is not painted on the inside, just a mill finish. They had terrible blisters completely through the cladding in less than 4 years.

[Jules]

Aha. That sounds like the stuff I have; it's certainly a lot less green than the P-T stuff I remember of a few years ago (enough that I was a little skeptical about it actually being treated at all when I picked it up due to the lack of any strong color ;)

According to the end-tags it's branded as MCA - Micronized Copper Azole (I never realised until now that the tags have a whole bunch of info on the reverse side - duh).

It still says to use hot-dipped galvanised fasteners, though. That's interesting because it is indeed the same stuff that Home Depot sells - yet I couldn't see any hot-dipped screws listed on HD's website (although I've found that their website's a mine of misinformation before, so who knows). There's a better supply place in town that I'm sure will do what I need, but I might swing by HD too just to see what their advice is; it'd be strange if they sell the wood but not the manufacturers' recommended fastners!

cheers

Jules

[cavedweller]

...more likely zinc plating with a "gold" dichromate rinse.

[Phisherman]

Most metal fasteners will corrode in treated lumber and discolor it. SS screws are best when using outdoors or in a humid environment.